This is a division of application Ser. No. 336,332 field February 27, 1973, now U.S. Pat. No. 3,850,000.
1. Field of the Invention
This invention relates to tunnelling shields for use in erecting a spiral tunnel lining.
2. Description of the Prior Art
Tunnelling shields for erecting spirally wound tunnel linings are known in which one or more turns of the spiral are erected in the rearward end of the shield and the shield is advanced by rams pushing on the end of the erected lining. In practice it has been found necessary to erect a number of segments and then advance the shield followed by a further erection operation so that the shield advances step by step between the operations of erecting segments. Furthermore on withdrawing the shield from around the lining a void is left which must be filled with grout or the wall of the tunnel could collapse against the lining and exert a nonuniform pressure on the lining which could cause the lining to collapse at that location. U.S. Pat. No. 739969 is representative of the above prior art.